Method of preserving fuel cell membrane electrode assembly

ABSTRACT

A method of preserving a fuel cell membrane electrode assembly in which catalyst electrodes are stacked on each surface of a polymer electrolyte is to preserve the fuel cell membrane electrode assembly in an airtight package that prevents oxygen, moisture and a function inhibitor from permeating through the package.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of preserving a fuel cellmembrane electrode assembly in which an air electrode and a fuelelectrode are respectively stacked onto the surfaces of a polymerelectrolyte.

2. Description of the Related Art

Recently, a fuel cell has attracted a great deal of attention as a cleanpower-generating resource. A variety of types of fuel cells areprovided, and a polymer-electrolyte-type fuel cell is among them.

The polymer-electrolyte-type fuel cell has a so-called fuel cell stack(tandem cell) in which a plurality of smallest units, each being calleda “unit cell” and generating power, are stacked in series. By providingthe fuel cell stack with a unit for providing oxygen and fuel or a unitfor cooling down the tandem cell, a desired power (voltage) can beobtained through a reaction between hydrogen and oxygen in each unitcell.

The unit cell has a separator for conducting electricity and separatingthe assemblies which are adjacent to each other when unit cells arestacked. It is the fuel cell membrane electrode assembly that mainlycontrols the reaction between hydrogen and oxygen.

The fuel cell membrane electrode assembly includes an electrolyte madeof a polymer ion-exchange membrane that is similar to a fluoropolymerion-exchange membrane that has a sulfonic group. The fuel cell membraneelectrode assembly also includes a cathode catalyst layer that becomesan air electrode and an anode catalyst layer that becomes a fuelelectrode when placed on each surface of the electrolyte. For example,metal made of platinum and ruthenium is used for the anode catalystlayer while platinum is used for the cathode catalyst layer.

The fuel cell membrane electrode assembly with the structure asdescribed above causes a reaction between oxygen and hydrogen asfollows: hydrogen gas provided for the fuel electrode is changed intohydrogen ion in the anode catalyst layer; and the hydrogen ion is moved,in a state of hydration, to the oxygen electrode's side through theelectrolyte. The ion then reacts with oxygen and electron and generateswater in the cathode catalyst layer. By repeating the reaction, the fuelcell membrane electrode assembly generates power (voltage).

Such a polymer electrolyte fuel cell is manufactured almost in sequencefrom a manufacturing of a unit cell, an assembling of a fuel cell stackthrough to a final process of assembling a fuel cell. Therefore, it ispossible to manufacture a fuel cell that has satisfactory functions.

Today, an era of mass production of fuel cells is about to coincide withthe spread of fuel cell use. There is a good possibility for a necessityto preserve, for a long time, without degrading the function thereof,the parts used for assembling a fuel cell in order to maintain thedesired functions throughout the process of manufacturing. For example,the following method is conceivable for preserving a fuel cell stack inan atmosphere that is purged of air (oxygen): purging with the use ofinert gas, or purging with the use of moisture, so that the air (oxygen)remaining in a fluid channel which is placed in a separator thattransmits oxygen gas and hydrogen gas can be eliminated (see referenceto Japanese Laid-Open Applications No. 2002-93448 and No. 06-251788).Another preservation method involving using an oxygen-absorbingsubstance is suggested as a technique of preserving a fuel cell stack inan atmosphere that is purged of oxygen (see reference to JapaneseLaid-Open Application No. 2000-289380).

It is possible to preserve the fuel cell stack for a long time, usingthe conventional method. Along with the progress in general use of fuelcells, however, in some cases, only fuel cell membrane electrodeassemblies are manufactured and transported to a distant place. In thiscase, the conventional method is not effective.

After diligent research through the years in view of the conventionaltechniques, the inventors of the present invention have comes, todiscover a cause of the problem generated in the preservation of fuelcell membrane electrode assembly.

According to the research, the cause of the problem turns out to be theuse of alcohol in the process of manufacturing a catalyst that makes upa membrane electrode assembly. For example, acetylene black carbonpowder that supports platinum-ruthenium metal particles or platinumparticles is used as a catalyst powder, and a pasty catalyst ismanufactured by dispersing this catalyst powder onto ethyl alcohol thatcontains perfluoro-carbon sulfone acid powder. The pasty catalyst isspread over a non-woven fabric made of carbon. A catalyst layer isformed in this way. A membrane electrode assembly is manufactured bysandwiching the electrolyte with two catalyst layers whose surface onwhich the catalyst is applied faces toward the electrolyte.

The alcohol remains, however, on the non-woven fabric even after themembrane electrode assembly is manufactured. If the membrane electrodeassembly is preserved in such condition, oxide is generated as a resultof the reaction between oxygen in the air and the alcohol, which affectsthe catalyst. The obtained observation is that the catalyst layer itselfmay be degenerated due to the long-term preservation of the membraneelectrode assembly.

Another observation is that, in some cases, a dust such as an organiccompound contained in the air may stick to the membrane electrodeassembly depending on the environmental condition in a factory or astock room, and the catalyst may be degenerated if an unnecessaryorganic compound adheres to the membrane electrode assembly for a longtime.

In the case where metallic (transition metal in particular) particulatesreach the electrolyte, the metal particles are ionized since theelectrolyte is strongly acid. When the electrolyte to which ionizedmetallic particles adhere is provided to the fuel cell so that the fuelcell is activated, hydroxyl radical is generated as a result of thereaction between hydrogen peroxide generated due to the gas that crossleaks from the electrolyte or the secondary reactions, and the ionizedmetal adhering to the metal particulates. The electrolyte is decomposedby the generated hydroxyl radical. The observation shows that, after thedecomposition of the electrolyte, the electrolyte increasingly crossleaks so as to accelerate the decomposition of the electrolyte resultingin decreases in film pressure of the electrolyte that are evident to theextent that power cannot be constantly generated.

It has also been observed that after the exposure to the oxygen in theair, each of the catalyst layers rises to a high voltage that is closeto 1V. This accelerates oxidization of a metallic catalyst such ascarrier carbon, platinum and ruthenium in the catalyst layer. Due to theoxidization, the catalyst layer loses its function as a catalyst or thecatalyst melts out of the catalyst layer which makes the layerdeficient.

Moreover, it turns out that the change in humidity in the environmentwhere the fuel cell membrane electrode assembly is preserved causesdamage to the electrolyte or to the catalyst layer after the repetitionof expansion and shrinking of the electrolyte.

The inventors also discovered that in the case where the fuel cellmembrane electrode assembly falls into one of the above cases, the fuelcell made of such fuel cell membrane electrode assembly can be a causeof degradation in initial characteristic such as voltage and/or currentcharacteristic or a cause of degradation in serviceability of the fuelcell over a long term.

It has also been found that the expansion and shrinking of theelectrolyte also causes change in size, which renders it difficult orimpossible to build up a unit cell.

Note that in the case where an oxygen-absorbing substance is placed in apackage that has low oxygen permeability, the problem of oxidization canbe prevented. However, the oxygen-absorbing substance must be carefullyselected because in some cases a substance that acceleratesdecomposition of electrolyte may be emitted from the oxygen-absorbingsubstance.

SUMMARY OF THE INVENTION

The present invention is conceived in view of the problems in the priorart and the above observations made by the inventors. An object of thepresent invention is to provide a method of preserving a fuel cellmembrane electrode assembly that can suppress the degradation in thecharacteristics of the fuel cell made of the fuel cell membraneelectrode assembly that has been preserved, even in the case where onlythe fuel cell membrane electrode assembly is preserved for a long periodof time.

In order to achieve the above object, a method according to the presentinvention of preserving fuel cell membrane electrode assembly having acatalyst electrode stacked on each surface of a polymer electrolyte,includes: preserving the fuel cell membrane electrode assembly in anairtight package that prevents oxygen, moisture and a function inhibitorfrom permeating through the package.

Thus, it is possible to maintain the atmosphere in the airtight packageafter the package is sealed, and prevent the degradation in thefunctions of the membrane electrode assembly and the adhesion ofunnecessary substances to the membrane electrode assembly.

According to the method, an atmosphere in the airtight package may havea lower oxygen concentration than air.

In this way, it is possible to prevent the damage to the catalyst layersand degradation of the catalyst layers which are caused by oxidizationof the membrane electrode assembly or oxidization of organic substancesthat remain in the membrane electrode assembly.

According to the method, a concentration of fuel gas in an atmosphere ofthe airtight package that has just been sealed may be higher than aconcentration of fuel gas in air.

This causes a reaction between the fuel gas and the residual oxygenthrough the catalyst in the membrane electrode assembly so that theairtight package is filled with the atmosphere that has little amount ofoxygen.

According to the method, a deoxidizer may be placed in the airtightpackage.

As a result, the airtight package can be easily filled with anatmosphere that has little amount of oxygen, and thereby, it is possibleto easily prevent the functions of the membrane electrode assembly frombeing degraded with time.

According to the method, a concentration of inert gas in an atmosphereof the airtight package may be higher than a concentration of inert gasin air.

Thus, a concentration of other gas such as oxygen can be relatively low,which makes it possible to obtain the same operational effects as can beobtained with the atmosphere that has a low oxygen concentration.

According to the method, the airtight package in which the fuel membraneelectrode assembly is placed may be sealed after an atmosphere of theairtight package is purged with preservative gas.

By applying this method, it is possible to preserve the membraneelectrode assembly in a desired atmosphere, and thereby prevent thefunctions of the membrane electrode assembly from being degraded due toa long-term preservation.

According to the method, the preservative gas may have a same degree ofhumidity as humidity inside the airtight package which has not yet beenpurged of oxygen.

Thus, the change in humidity can be suppressed even at an earlier timeof the preservation of the membrane electrode assembly, and the changein size of the membrane electrode assembly at an initial stage can beprevented as well. This prevents the functions such as initialcharacteristics and serviceability from being degraded due to along-term preservation of the membrane electrode assembly.

According to the method, the airtight package in which the membraneelectrode assembly is preserved is sealed after an atmosphere of theairtight package is filled with preservative gas.

With the method described above, it is possible to preserve the membraneelectrode assembly in a desired atmosphere within the airtightcontainer, and thereby to suppress the function degradation caused by along-term preservation.

According to the method, a degree of humidity in the preservative gasatmosphere may be as same as a humidity of an atmosphere around the fuelcell membrane electrode assembly that has not yet been preserved in theairtight package

Thus, the change in humidity can be suppressed even at an earlier timeof the preservation of the membrane electrode assembly, and the changein size of the membrane electrode assembly at initial stage can beprevented as well. This prevents the functions such as initialcharacteristics and serviceability from being degraded due to along-term preservation of the membrane electrode assembly.

According to the method, an amount of oxygen permeated through theairtight package is 0.1 ml/(m²/day/atm) or below and moisturepermeability is 0.1 g/(m²/day) or below.

Thus, it is possible to specify the characteristics of the airtightcontainer that enable the membrane electrode assembly to be preservedover a long period of time.

According to the method, a surface of the catalyst electrode of the fuelcell membrane electrode assembly may be covered with a protective filmthat has high oxygen barrier properties

Thus, the contact between the membrane electrode assembly and the oxygencan be directly interrupted, and moreover, the degradation of thefunctions such as initial characteristics and serviceability caused bythe long-term preservation of the membrane electrode assembly can beprevented. Furthermore, a cushioning function of the film can preventdeficiencies in the membrane electrode assembly caused by shocks givenfrom outside or a contact between the membrane electrode assemblies.

FURTHER INFORMATION ABOUT TECHNICAL BACKGROUND TO THIS APPLICATION

The disclosure of Japanese Patent Applications No. 2004-065196 filed onMar. 9, 2004 and No. 2004-245566 filed on Aug. 25, 2004, includingspecification, drawings and claims is incorporated herein by referencein its entirety.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects, advantages and features of the invention willbecome apparent from the following description thereof taken inconjunction with the accompanying drawings that illustrate a specificembodiment of the invention. In the Drawings:

FIG. 1 is a schematic diagram showing a conventional method ofpreserving a fuel cell stack;

FIG. 2 is a perspective view showing a condition in which a membraneelectrode assembly is preserved according to embodiments of the presentinvention;

FIG. 3 is a cross-sectional view showing a condition in which a membraneelectrode assembly is preserved according to the embodiments of thepresent invention;

FIG. 4 is a cross-sectional view showing a condition in which pluralmembrane electrode assemblies are preserved in a single airtightpackage, according to the first embodiment;

FIG. 5 is a cross-sectional view showing a condition in which a membraneelectrode assembly covered with a protective film is preserved accordingto a second embodiment;

FIG. 6 is a cross-sectional view showing a condition in which pluralmembrane electrode assemblies, each being covered with a protectivefilm, are preserved in a single airtight package, according to thesecond embodiment;

FIG. 7 is a sketch of an apparatus for purging inside the airtightpackage with the use of preservative gas, according to the secondembodiment;

FIG. 8 is a sketch of an apparatus for filling the airtight package withpreservative gas, according to the second embodiment;

FIG. 9 is a cross-sectional view showing a condition for rendering theairtight package to have the atmosphere with low oxygen concentration,according to the second embodiment; and

FIG. 10 is a graph showing how cell voltages of different groups varywith time.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following describes the embodiments of the present invention withreference to the diagrams.

First Embodiment

FIG. 2 is a perspective view showing a condition in which a fuel cellmembrane electrode assembly is preserved according to the embodiments ofthe present invention.

FIG. 3 is a cross-sectional view in the case of virtually cutting amembrane electrode assembly 11 preserved in an airtight package 21.

As shown in FIGS. 2 and 3, the membrane electrode assembly 11 ispreserved in the airtight package 21 that is a bag made of resin.

The membrane electrode assembly 11 is a fuel cell membrane electrodeassembly with a structure in which an anode catalyst layer 13 and acathode catalyst layer 14 are stacked respectively on each surface of apolymer electrolyte 12. Each of the catalyst layers 13 and 14 has astructure in which carbon mesh supports the catalyst, and is vulnerableagainst shocks. As shown in detail in FIG. 3, a gas diffusion layer 15is placed on one of the surfaces of the respective catalyst layers 13and 14.

The airtight package 21 is made up of materials with high sealingproperties for preventing oxygen, moisture and function inhibitors frombeing transmitted. The airtight package 21 may be a stiff container witha stable form, however, a bag made of a flexible polymeric film with lowmoisture permeability and low oxygen permeability is more preferablebecause such bag can be preserved without taking up much space when itis not used for the preservation of membrane electrode assembly 11.

More precisely, the airtight package 21 needs moisture permeability of0.1 g/m²/day or less and oxygen permeability of 0.1 cc/m²/day/atm orless. This is because degradation with time of the membrane electrodeassembly 11 becomes serious unless both of the conditions are satisfied.It is desirable if moisture permeability is 0.01 g/m²/day or less andoxygen permeability is 0.01 cc/m²/day/atm or less. With the conditionssatisfied, the airtight package 21 can serve for a long-termpreservation.

Material can be selected for the airtight package 21 from the following:a plastic film or a ceramic evaporated film, of polyvinyl chloride(PVDC), of ethylene-vinylalcohol (EVOH), of polyvinyl alcohol (PVA) andof polyamide (PA), a single aluminum evaporated film, a single aluminumfoil laminated film, or a film made by laminating plural barriermaterials, or a film made by compounding the barrier materials used fora barrier layer and a polymeric film.

Representative examples of the polymeric film are the following: a PVDCcoat OPP, a PVA coat OPP, an EVOH co-extruded OPP, a PVDC coat ONY, amultilayered barrier ONY (MXD, an EVOH co-extruded), a PVDC coat PET, aPVA coat PET, a PVDC coat cellophane, an EVOH film, an extensible PVAfilm, a hybrid barrier coat film, an alumina evaporated film (an alumina(Al203) evaporated PET, a silica (SiOx) evaporated PET, an aluminaevaporated ONY, an alumina evaporated OPP), an aluminum evaporated film(aluminum evaporated PET, an aluminum evaporated CPP, an aluminumevaporated OPP, an aluminum evaporated ONY and an aluminum evaporatedPE), or the like.

Note that OPP stands for bi-oriented propylene, ONY stands forbiaxially-oriented nylon, and CPP signifies inextensible propylene whileMXD denotes polyamide resin with barrier properties and PET denotespoly-ethylene terephthalate.

The thickness of the airtight package 21 is not strictly specified, butit only requires the above materials and to be thick enough to retainmoisture permeability of 0.1 g/m²/day or less and oxygen permeability of0.1 cc/m²/day/atm or less. Vapor depositing aluminum onto the film madeof the above materials can surprisingly decrease moisture permeabilityas well as oxygen permeability, which makes the airtight package 21thinner.

By using the airtight package 21 made of the above-mentioned material soas to retain moisture permeability of 0.1 g/m²/day or less, it ispossible to prevent the vapor from leaking out of the airtight package21, and thus, to avoid change in amount of moisture inside the airtightpackage 21. However, when rapid change in temperature is generatedoutside the airtight package 21, dropwise condensation is generated andhumidity changes greatly in the airtight package 21. It is thereforeadvisable that the membrane electrode assembly 11 be preserved in aplace, e.g., a hygrostat and temperature-controlled bath wheretemperature can be maintained within a predetermined range, but not in aplace where temperature greatly fluctuates. In this way, it is possibleto prevent the change in the amount of moisture in the polymerelectrolyte as well as the degradation of the polymer electrolyte.

The use of the airtight package 21 can also prevent an inclusion offunction inhibitors from outside as well as decrease in functions of thefuel cell caused by the function inhibitor adhering to the membraneelectrode assembly 11.

Here, function inhibitors are, for instance, a metallic element such asan organic substance, iron and transition metal, and metallic fineparticles. Such function inhibitors include not only a substance thatdegrades the functions of the membrane electrode assembly 11 during along-term preservation, but also a substance that decreases the functionas a fuel cell by adhering to the membrane electrode assembly 11.

Note that in preserving the membrane electrode assembly 11 without thecatalyst layers 13 and 14 (i.e. at the stage where the assembly 11includes only the polymer electrolyte 12 under the process of assembly),it is possible to prevent the contact between the polymer electrolyte 12and oxygen or function inhibitors so as to maintain the amount ofmoisture, by preserving the polymer electrolyte 12 covered with aprotective film 16 in the airtight package 21. It is thus possible toprevent the decrease of functions of polymer electrolyte 12 even in thecase of preserving the polymer electrolyte 12 for a long period of time.

FIG. 4 is a cross-sectional view showing the case of preserving pluralfuel cell membrane electrode assemblies 11 in such manner that they arestacked.

In the airtight package 21, the plural membrane electrode assemblies 11are sealed in such manner that they are stacked whereas gas diffusionlayers 15 are stacked on the catalyst layers 13 and 14 such that a gasdiffusion layer 15 of one membrane electrode assembly 11 is in contactwith a gas diffusion layer 15 of another membrane electrode assembly 11.

Thus, it is also possible to preserve plural membrane electrodeassemblies 11 all together in the airtight package 21.

By preserving the membrane electrode assembly 11 as described above, itis possible to preserve a single fuel cell membrane electrode assembly11 in the airtight package 21 which prevents oxygen, moisture andfunction inhibitor from being transmitted under the condition where theatmosphere is purged of oxygen and where change in the amount ofmoisture (humidity) in the airtight package 21 is suppressed. Thistherefore leads to the prevention of decrease in power-generatingfunction during the preservation of the fuel cell membrane electrodeassembly 11, which does not cause degradation in the function of themembrane electrode assembly 11 over a long period.

It is also possible to fill the airtight package 21 with water. Theoxygen can be eliminated out of the airtight package 21 by pouring waterinto the airtight package 21. As the membrane electrode assembly 11 issurrounded by water, change in amount of moisture in the polymerelectrolyte 12 can be prevented. It is desirable to use purified ordistilled water for the filling because it does not contain any functioninhibitors.

Second Embodiment

The following describes the second embodiment with reference to thediagrams.

FIG. 5 is a cross-sectional view showing the condition in which themembrane electrode assembly 11, whose surface is covered with theprotective film 16, is preserved.

The membrane electrode assembly 11 and the airtight package 21 are thesame as those described in the first embodiment, therefore, thedescription is not repeated here.

The protective film 16 plays a role of directly preventing the contactbetween the membrane electrode assembly 11 and oxygen or functioninhibitors that remain in the airtight package 21. The protective film16 also serves as a cushioning medium to prevent the damage to thesurface of the catalyst. A polymer film resin with high barrierproperties against oxygen is used as a material for the protective film16. Note that the same material as used for the airtight package 21 maybe used for the protective film 16.

The membrane electrode assembly 11 has a structure in which the anodecatalyst layer 13 and the cathode catalyst layer 14 are stacked on eachsurface of the polymer electrolyte 12. The gas diffusion layer 15 isplaced on each of the catalyst layers 13 and 14.

Even in the case where the membrane electrode assembly 11 has the gasdiffusion layers 15, the contact between the surface of the catalyst andoxygen or function inhibitors is further reduced, and change in theamount of moisture on the surface of the catalyst further decreases aswell, by covering the surface with the protective film 16. The coveringhas a purpose to allow the gas diffusion layers to serve as protectivefilms, which further prevents the degradation of the power-generatingfunction of the membrane electrode assembly 11 due to the long-termpreservation. Therefore, it is desirable to preserve the membraneelectrode assembly 11 that includes the gas diffusion layer 15.

FIG. 6 is a cross-sectional view showing the case of preserving pluralmembrane electrode assemblies 11 in a single airtight package 21.

In the airtight package 21, plural membrane electrode assemblies 11 aresealed in such manner that they are stacked on each other. By coveringthe surface of each of the membrane electrode assemblies 11 with theprotective film 16, it is possible to preserve them in a single airtightpackage 21 so that the membrane electrode assemblies 11 contact eachother without being damaged.

Note that in the present embodiment, each membrane electrode assembly 11has a gas diffusion layer 15, however, in some cases, the membraneelectrode assembly 11 without the gas diffusion layer 15 may bepreserved.

Method 1 for Creating an Atmosphere of the Airtight Package 21

The following describes a method for creating the atmosphere that haslow oxygen concentration in the airtight package 21.

The membrane electrode assembly 11 is inserted in a bag-type airtightpackage 21 that is party opened.

As shown in FIG. 7, an air release pipe 31 that exhausts gas from theairtight package 21 and a supply pipe 32 that provides preservative gasare inserted, while an opening of the airtight package 21, in which themembrane electrode assembly 11 is inserted, is held down.

The air release pipe 31 is connected to a gas exhaust apparatus 33, andopens or closes with an air release valve 34 while the supply pipe 32 isconnected to a gas supply apparatus 35, and opens or closes with asupply valve 36.

Then, the air release valve 34 is released so that the air in theairtight package 21 is introduced to outside and the airtight package 21is vacuumed. After this, the air release valve 34 and the supply valve36 are switched to be closed, and the airtight package 21 is filled withpreservative gas.

Lastly, the opening is heat-shielded at the same time when the pipes 31and 32 are pulled out so that the airtight package 21 is sealed off.

Note that pipes to be used for the air release pipe 31 and the supplypipe 32 may be same or different. In the case of using the same pipe, avalve for switching a line for gas release to a line for gas supply isused.

The preservative gas only requires gas with low oxygen concentration,and it is preferable that the gas has inert gas as a main component. Thegas may contain fuel gas, but still has inert gas as a main component.

The fuel gas may be represented by hydrogen gas, and is provided to theanode's side in the fuel cell.

In the case where the preservative gas includes such fuel gas, the fuelgas included in the preservative gas and a slight amount of oxygen gasthat remains in the airtight package 21 or that comes into the airtightpackage 21 from outside are used for a burning reaction generated in thecatalyst layer in the membrane electrode assembly 11. Therefore, neitherthe oxygen gas remains in the airtight package 21, nor the catalystlayer is maintained at high level voltage due to the oxidization of themembrane electrode assembly 11 or oxygen. This prevents degradation ofthe functions of the membrane electrode assembly 11.

The inert gas includes nitrogen, but may include helium and argoninstead.

Furthermore, the airtight package 21 may be filled with inert gas thatincludes fuel gas based on the following methods: encapsulating fuel gasafter purging the atmosphere with inert gas; and purging the atmospherewith inert gas after encapsulating fuel gas. Here, the respective gascan be encapsulated into the airtight package 21 by separately operatinga pipe for encapsulating inert gas and a pipe for encapsulating fuelgas, by switching between the valves.

By allowing the preservative gas to have the same degree of humidity ashumidity of the condition in which the membrane electrode assembly 11 ismanufactured, it is possible to prevent change in the degree of humidityat the time of gas purging.

It is described that the opening is sealed based on the method of thermocompression bonding, however, a method based on zipping or a methodbased on compression may be applied instead. Note that the method basedon thermo compression bonding is desirable for its excellent sealingproperties and easy process.

In this way, the atmosphere inside the airtight package 21 can be purgedwith preservative gas, which can fill the airtight package 21 with adesirable atmosphere.

Method 2 for Creating an Atmosphere in the Airtight Package 21

The following describes another method of filling the airtight package21 with the atmosphere that has low oxygen condensation.

First, the membrane electrode assembly 11 is inserted into the bag-typeairtight package 21 a part of which is opened.

Then, the airtight package 21, in which the membrane electrode assembly11 is inserted, is placed in a large chamber 51 as shown in FIG. 8.

The fuel cell membrane electrode assembly 11 covered with the protectivefilm 16 is inserted into the airtight package 21. The membrane electrodeassembly 11 may include the diffusion layer 15 or may be at a stagewhere only the catalyst layers 13 and 14 are stacked onto the polymerelectrolyte 12.

The airtight package 21 may be made of metal or resin that preventsmoisture and function inhibitors from entering from outside.

The chamber 51 includes the air release pipe 31 for deaerating the gaswithin the chamber 51, and the supply pipe 32 for providing preservativegas such as inert gas and fuel gas. The on-off valves 34 and 36 arerespectively connected to the air release pipe 31 and the supply pipe 32so that gas release and gas supply can be arbitrarily performed. The airrelease pipe 31 and the supply pipe 32 may be the same pipe. In thiscase, the connection to the air release pipe 31 and the supply pipe 32may be changed using the valves.

Then, the on-off valves 34 and 36 are controlled so as to provide thechamber 51 with inert gas that contains fuel gas, after the chamber 51is vacuumed by vacuuming the air. The pressure after the provision ofinert gas is assumed to be 1 atm.

Lastly, the opening of the airtight package 21 is sealed within thechamber 51 using the thermo compression bonding method or the like, sothat the air is vacuumed.

This method is preferable since plural membrane electrode assemblies 11can be simultaneously sealed in the chamber 51 filled with thepreservative gas atmosphere according to the capacity of the chamber 51.Even in the case where air is vacuumed out of the airtight package 21,it is possible to prevent the damages to the catalyst layers 13 and 14caused by the atmospheric pressure, because the atmospheric pressuredoes not directly affect the membrane electrode assembly 11.

Method 3 for Creating an Atmosphere in the Airtight Package 21

FIG. 9 is a cross-sectional view showing the condition in which themembrane electrode assembly 11 is preserved in the airtight package 21in which a deoxidizer 3 is placed.

In the case of using the deoxidizer 3, the deoxidizer 3 is put into theairtight package 21 so that the airtight package 21 is sealed off in theair.

Thus, with the use of deoxidizer 3, it is easy to fill the airtightpackage 21 with the atmosphere that has low oxygen condensation.

In the case of using the deoxidizer 3, it is also possible toselectively exclude the oxygen that leaks from outside. Even in the casewhere the airtight package 21 has a low degree of closure, theatmosphere in the airtight package 21 can be continuously purged ofoxygen for a predetermined period of time.

It is desirable to use an auto-reactive, organic deoxidizer. Suchdeoxidizer does not require moisture from outside, therefore, canprevent change in amount of moisture within the airtight package 21. Incontrast, an iron deoxidizer rapidly absorbs oxygen and is economic interms of cost, however, has deficiencies in depending on moisture andcausing change in amount of moisture within the airtight package 21. Itis therefore desirable not to use iron for fuel cells since it becomes afunction inhibitor that degrades the power-generating function equippedin the fuel cell.

EXAMPLE

The following describes, with reference to FIG. 10, the result ofexamining a possibility to prevent the decrease in the power-generatingfunction as well as the decrease of the serviceability which areequipped in the membrane electrode assembly 11 that is preserved basedon the preservation method according to the present invention.

Firstly, the number of membrane electrode assemblies 11 as many as theycan form at least three fuel batteries was provided. A group A coveredeach of the membrane electrode assemblies 11 with an EVOH film and thenleft, for a year, the assemblies 11 in the airtight package 21 filledwith an inert gas atmosphere. A group B left, for a year, the membraneelectrode assemblies 11 in the airtight package 21 filled with the inertgas atmosphere. A group C wrapped each of the membrane electrodeassemblies 11 with a polyethylene sheet and left the assemblies 11 for ayear. After that, each of the membrane electrode assemblies 11classified by each group is stacked together with a separator so as tomake a fuel cell.

In the examination, hydrogen gas is provided as a fuel for an anodeelectrode while air is provided for a cathode electrode as a gas thatcontains an oxidizer. The conditions are as follows: the temperature ina cell is 70□; fuel utilization is 70%; oxygen utilization is 40%; andpower flux density is 0.2 A/cm².

FIG. 10 shows how cell voltages of different groups vary with time untilthe operating time reaches 3000 hours. Note that a broken line in thediagram indicates, for comparison, the result of generating power in afuel cell that has just been manufactured.

According to the observation, at initial stage, cell voltage of themembrane electrode assemblies 11 (A) and (B) that are preserved based onthe preservation method according to the present invention is higherthan that of the membrane electrode assembly 11 (C) that is preservedbased on the conventional preservation method, and is almost as same asthe cell voltage in the case of generating power in the membraneelectrode assembly 11 that has just been manufactured. With regard tothe voltage decay rate until the operation time reaches 3000 hours, thevoltage decay rate of the cell voltage based on the method according tothe present invention is lower than that of the cell voltage based onthe conventional preservation method, and is almost as same as thevoltage decay rate of the cell voltage in the case of generating powerin the membrane electrode assembly 11 that has just been manufactured.In the case of further continuing, for a long time, the power generationin the cell, for which the membrane electrode assembly 11 that ispreserved based on the conventional preservation method is used,compared to the case of generating power in the cell for which themembrane electrode assembly 11 that has just been manufactured is used,the following results are obtained: the degradation of polymerelectrolyte progresses faster than usual; and the voltage rapidlydecreases in a very short time, which causes inability to generatepower. It is also verified that the cell voltage can be alwaysmaintained at higher level in the group A which covered the membraneelectrode assembly 11 with an EVOH film than the group B which didn't.

Therefore, the preservation method according to the present inventioncan prevent decrease in terms of both power-generating function andserviceability of the membrane electrode assembly 11 caused by long-termpreservation better than the conventional preservation method. The sameeffects are obtained in the case where the membrane electrode assembly11 is left under the environmental condition that ordinary temperatureis between 20 to 30 degrees Celsius, high temperature is between 50 to60 degrees Celsius, and low temperature is between 0 to 10 degreesCelsius.

Although only some exemplary embodiments of this invention have beendescribed in detail above, those skilled in the art will readilyappreciate that many modifications are possible in the exemplaryembodiments without materially departing from the novel teachings andadvantages of this invention. Accordingly, all such modifications areintended to be included within the scope of this invention.

1. A method of preserving a fuel cell membrane electrode assembly inwhich catalyst electrodes and a polymer electrolyte are arranged in alayered manner, the method comprising: placing the fuel cell membraneelectrode assembly in an airtight package that prevents oxygen, moistureand a function inhibitor from permeating through the package, purging anatmosphere of the airtight package of oxygen with a preservative gashaving a same degree of humidity as a humidity inside the airtightpackage prior to said purging, and sealing the airtight package aftersaid purging.
 2. The method of preserving a fuel cell membrane electrodeassembly, according to claim 1, wherein said purging includes thepreservative gas having a lower oxygen concentration than ambient air.3. The method of preserving a fuel cell membrane electrode assembly,according to claim 2, wherein said purging includes purging such that aconcentration of fuel gas in the atmosphere of the airtight packageafter sealing is higher than the concentration of fuel gas in ambientair.
 4. The method of preserving a fuel cell membrane electrodeassembly, according to claim 2, further comprising placing a deoxidizerin the airtight package.
 5. The method of preserving a fuel cellmembrane electrode assembly, according to claim 1, wherein said purgingincludes purging such that a concentration of inert gas in theatmosphere of the airtight package after sealing is higher than theconcentration of inert gas in ambient air.
 6. The method of preserving afuel cell membrane electrode assembly, according to claim 1, whereinsaid purging includes the preservative gas having inert gas as a maincomponent.
 7. The method of preserving a fuel cell membrane electrodeassembly, according to claim 1, wherein said purging includes thepreservative gas having fuel gas as a component.
 8. The method ofpreserving a fuel cell membrane electrode assembly, according to claim1, wherein said sealing includes sealing such that an amount of oxygenpermeated through the airtight package is 0.1 ml/(m²/day/atm) or lessand moisture permeability of the airtight package is 0.1 g/(m²/day) orless.
 9. The method of preserving a fuel cell membrane electrodeassembly, according to claim 1, further comprising covering a surface ofthe catalyst electrode of the fuel cell membrane electrode assembly witha protective film having barrier properties against oxygen.
 10. Themethod of preserving a fuel cell membrane electrode assembly, accordingto claim 9, further comprising laying a gas diffusion layer between thecatalyst electrode and the protective film in the fuel cell membraneelectrode assembly.
 11. A method of preserving a fuel cell membraneelectrode assembly in which catalyst electrodes and a polymerelectrolyte are arranged in a layered manner, the method comprising:placing the fuel cell membrane electrode assembly in an airtight packagethat prevents oxygen, moisture and a function inhibitor from permeatingthrough the package, purging an atmosphere of the airtight package witha preservative gas having a degree of humidity the same as the humidityof an atmosphere around the fuel cell membrane electrode assembly beforesaid placing in the airtight package, and sealing the airtight packageafter said purging.
 12. The method of preserving a fuel cell membraneelectrode assembly, according to claim 11, wherein said purging includesthe preservative gas having inert gas as a main component.
 13. Themethod of preserving a fuel cell membrane electrode assembly, accordingto claim 11, wherein said purging includes the preservative gas havingfuel gas as a component.
 14. The method of preserving a fuel cellmembrane electrode assembly, according to claim 11, wherein sealingincludes sealing such that an amount of oxygen permeated through theairtight package is 0.1 ml/(m²/day/atm) or less and moisturepermeability of the airtight package is 0.1 g/(m²/day) or less.
 15. Themethod of preserving a fuel cell membrane electrode assembly, accordingto claim 11, further comprising sealing a surface of the catalystelectrode of the fuel cell membrane electrode assembly with a protectivefilm having barrier properties against oxygen.
 16. The method ofpreserving a fuel cell membrane electrode assembly, according to claim15, further comprising laying a gas diffusion layer between the catalystelectrode and the protective film in the fuel cell membrane electrodeassembly.